The corrosion of metallic materials represents a problem which has today still not been satisfactorily solved. As a result of the corrosion, by which is meant the generally electrochemical reaction of a metallic material with its atmospheric surroundings, more particularly oxygen and water, there are significant alterations to the material. Corrosion damage leads to impairment of function in metallic components and ultimately to the need for the components to be repaired or replaced. The corresponding economic significance of corrosion, and of protection against corrosion, is therefore highly relevant.
It is for these reasons that great importance is accorded to corrosion control across virtually all sectors of the metal industry (examples being mechanical engineering and equipment, automotive industry (vehicle construction), aviation and aerospace industry, shipbuilding industry, electrical industry, precision mechanics industry), but especially in the sectors of the automotive and aviation industries. In the latter sectors in particular, metallic substrates are used very extensively as components, which are exposed to atmospheric conditions, in some cases to extreme atmospheric conditions.
In the finishing of vehicles and in the aviation industry, metallic substrates are typically subjected to an expensive and involved multicoat coating procedure. This is necessary in order to be able to meet the exacting requirements of the vehicle-making and aviation industries—which include effective corrosion control, for example.
Commonly first of all as part of the pretreatment of the metallic substrate, a conversion coating is constructed that protects against corrosion. Examples include the phosphatizing of steel substrates or chromating of aluminum substrates or aluminum alloys, examples being specialty aluminum-copper alloys such as the AA2024-T3 alloy. The latter finds application primarily in the aviation industry on account of its very good processing properties, its low density and at the same time resistant nature with respect to physical stressing. At the same time, however, the material has a propensity toward the hazardous filiform corrosion, where, often after physical damage to the substrate coating in conjunction with high atmospheric humidity, the corrosion propagates in filament form beneath the coating of the substrate and produces filiform corrosion damage to the metallic substrate. Effective corrosion control, accordingly, is important.
Following the pretreatment and the construction of appropriate conversion coats, in principle a primer coat is produced which provides protection from corrosion. This primer coat is based on an organic-polymeric matrix and may further comprise the anticorrosion pigments that are described later on below. In the context of the automotive industry, this primer coat generally constitutes an electrodeposition coating, more particularly a cathodic electrocoat. In the aircraft industry, special epoxy resin-based primers are usually employed. In the automotive finishing sector, what then follows, generally, is the production of a surfacer coating, whose function, for example, is to compensate any unevennesses still present in the substrate, and to protect the cathodic electrocoat from stonechip damage. In the last step, finally, the topcoat is applied, which particularly in the case of automotive finishing is composed of two separately applied coats, a basecoat and a clearcoat.
One effective form of corrosion protection of metallic substrates, and one which is also still used nowadays, is the use of chromates. Chromates are used, for example, in the construction of conversion coats as part of the surface pretreatment of metallic substrates (chromating). Frequently, likewise, chromates are used as anticorrosion pigments directly in anticorrosion primers based on organic-polymeric resins. These primers, therefore, are coating materials or paints which in addition to known film-forming components such as organic resins, as binder, further comprise certain chromates in the form of chromate salts (e.g., barium chromate, zinc chromate, strontium chromate).
The corrosion control effect of chromates, in the construction of conversion coats by the etching of the metallic surface (aluminum, for example) and the consequent proportional reduction of the chromate to form trivalent chromium, for example, and also the construction of low-solubility passivation coats of mixed aluminum(III)/chromium(III)/chromium(VI) oxide hydrates, has been known for a long time.
Problems, however, are presented by the high toxic and carcinogenic effect of the chromates, and the associated burden on people and the environment. Avoiding chromates in the vehicle industry while at the same time retaining appropriate protection from corrosion has therefore long been a desideratum within the corresponding branches of industry.
An example of one possible approach for avoiding chromates while at the same time retaining an appropriate protection from corrosion is the use of oxo anions (and/or salts thereof) of various transition metals, such as MoO42−, MnO4− and VO3−, for example. Also known is the use of lanthanoid cations or different organic species such as, for example, benzotriazoles, ethylenediaminetetraacetic acid (EDTA), quinoline derivatives or phosphate derivatives. The underlying mechanisms of action are complex and even now are still not fully understood. They range from the formation of passivating oxide/hydroxide coats on the corroding metal surface through to the complexation of certain metal cations (Cu(II), for example) and the associated suppression of specific forms of corrosion (an example being the filiform corrosion of aluminum-copper alloys).
A further approach lies in the use of so-called nanocontainer materials and/or layer structure materials such as, for example, organic cyclodextrins or inorganic materials such as zeolites, alumina nanotubes and smectites. Also in use are hydrotalcite components and layered double hydroxide materials. The latter are usually referred to in the general technical literature together with the corresponding abbreviations “LDH”. In the literature they are frequently described by the idealized general formula [M22+(1-x)M33+x(OH)2]x+[Ay−(x/y)nH2O] or similar empirical formulae. In these formulae, M2 stands for divalent metallic cations, M3 for trivalent metallic cations, and A for anions of valence x. In the case of the naturally occurring LDH these are generally inorganic anions such as carbonate, chloride, nitrate, hydroxide and/or bromide. Various further organic and inorganic anions may also be present more particularly in synthetic LDH, which are described later on below. The general formula above also accounts for the water of crystallization that is present. In the case of the hydrotalcites, the divalent cation is Mg2+, the trivalent cation is Al3+, and the anion is carbonate, although the latter may be substituted at least proportionally by hydroxide ions or other organic and also inorganic anions. This is true especially of the synthetic hydrotalcites. The hydrotalcites can therefore be identified as a special form of the layer structures known generally as LDH. The hydrotalcites and LDH have a layerlike structure similar to that of brucite (Mg(OH)2), in which between each pair of metal hydroxide layers, which are positively charged because of the trivalent metal cations proportionally present, there is a negatively charged layer of intercalated anions, this layer generally further containing water of crystallization. The system is therefore one of layers with alternating positive and negative charges, forming a layer structure by means of corresponding ionic interactions. In the formula shown above, the LDH layer structure is accounted for by the brackets placed accordingly.
Between two adjacent metal hydroxide layers it is possible for various agents to be intercalated, examples being the anticorrosion agents referred to above, by means of noncovalent, ionic and/or polar interactions. For instance, in the case of the hydrotalcites and LDH, anticorrosion agents in anionic form are intercalated into the anionic layers. They are incorporated directly into corresponding coating materials based on polymeric binders (primers, for example) and hence contribute to the corrosion control. In this case they support the conversion coats that provide protection against corrosion. Attempts are also being made to replace the conversion coats completely, in which case the corresponding primers are then applied directly to the metal. In this way, the coating procedure is made less involved and hence more cost-effective.
WO 03/102085 describes synthetic hydrotalcite components and layered double hydroxides (LDH) comprising exchangeable anions and the use thereof in coating materials for the purpose of improving the corrosion control on aluminum surfaces. The layered double hydroxides here are described by the idealized general formula [M22+(1-x)M33+x(OH)2]x+[Ax−nH2O] already indicated earlier on above. Preferred metal cations are the hydrotalcite cations magnesium(II) and aluminum(III). Anions described are, for example, nitrate, carbonate or molybdate, but also the chromium-containing anions chromate and dichromate, with the toxic, carcinogenic chromate exhibiting the best corrosion control.
Further hydrotalcite components and LDH and the use thereof as anticorrosion agents in coating materials based on organic polymeric binders are described in EP 0282619 A1, WO 2005/003408 A2 or ECS Transactions, 24 (1) 67-76 (2010), for example. In these cases, as well as the inorganic anions described already, there are also organic anions used, for example, such as salicylate, oxalate, DMTD (2,4-dimercapto-1,3,4-thiadiazole) and derivatives thereof, anions obtainable from EDTA, or benzotriazolate.
In spite of the approaches described above, the problem of corrosion has to date not been satisfactorily solved. A consequence of this is that, even now, it is still necessary to use chromium-containing compounds widely as anticorrosion agents in order to guarantee appropriate corrosion control.
WO 2009/062621 A1 likewise describes the use of LDH in coating materials for producing stonechip-resistant OEM coating systems in the automotive finishing field. As is known, such OEM coating systems are composed of an anticorrosion coating (more particularly cathodic electrocoat), a primer-surfacer coat, a basecoat, and a clearcoat to finish. The LDH are used in the primer-surfacer coat. This primer-surfacer coat is said to exhibit not only high stonechip resistance but also effective adhesion to the underlying cathodic electrocoat and to the basecoat lying on top, and, furthermore, good surfacing properties (masking the structure of the substrate). The use of LDH components as corrosion inhibitors is not described. Nor is there a description of application of the coating materials comprising LDH components directly to the substrate. Organic anions employed include, for example, m- or p-aminobenzenesulfonate, m- or p-hydroxybenzenesulfonate, m- or p-aminobenzoate and/or m- or p-hydroxybenzoate.